patch 9.1.0034: Window scrolls unexpectedly when 'scrollbind' is set

Problem:  Window may unexpectedly scroll when 'scrollbind' is set
          and setting a buffer-local option using setbufvar()
          (Boris Staletic)
Solution: Save and restore the windows topline before opening the
          popup window.

fixes: #13863
closes: #13869

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index c20ff48..a397f70 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -4215,4 +4215,22 @@
   set maxfuncdepth&
 endfunc
 
+func Test_popupwin_setbufvar_changing_window_view()
+  " Test for Github Issue https://github.com/vim/vim/issues/13863
+  " using setbufvar(buf, '&option') should not scroll
+  " the current window
+  20new
+  call append(0, range(1, 25))
+  setlocal scrollbind
+  norm! G
+  let topline = winsaveview()['topline']
+  call setbufvar(winbufnr(popup_atcursor(['foobar'], {})), '&syntax', 'python')
+  " close popup
+  call popup_clear()
+  call assert_equal(topline, winsaveview()['topline'])
+
+  " clean up
+  bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2